1. Field of the Invention
The present invention relates to a zoom lens which is suitable for use in a broadcasting television camera, a video camera, a digital still camera, and a silver-halide film camera, and to an image pickup apparatus including the zoom lens.
2. Description of the Related Art
In recent years, there has been demanded a zoom lens having a high aperture ratio, a high zoom ratio, and high optical performance for an image pickup apparatus such as a broadcasting television camera, a silver-halide film camera, a digital still camera, and a video camera.
Positive-lead, telephoto type four-unit zoom lenses in which four lens units are provided in total and one of the lens units located closest to an object side has positive refractive power have been known as zoom lenses having a high zoom ratio.
For instance, there has been known a four-unit zoom lens including a first lens unit having positive refractive power for focusing, a second lens unit having negative refractive power for varying magnification, a third lens unit having negative refractive power for correcting image plane variation, and a fourth lens unit having positive refractive power for imaging.
In general, in order to achieve a higher zoom ratio, if a focal length at a telephoto end is set longer, an axial chromatic aberration among various aberrations at a telephoto side increases. If a focal length at a wide-angle end is set shorter, a lateral chromatic aberration among various aberrations at a wide-angle side increases. It is important to appropriately correct the chromatic aberration not only in a first-order spectrum but also in a second-order spectrum for obtaining high-quality image performance.
As a method of reducing such occurrence of the chromatic aberration, an achromatization method using an optical material having extraordinary dispersion and an achromatization method using a diffraction grating have been known well in general (see Japanese Patent Application Laid-Open No. 2006-349947).
In addition, there has been a demand for change in optical performance, in particular smaller focus change, even if ambient temperature varies.
There has been known a technology capable of easily changing a magnification-varying range in a zoom lens for a television camera by adopting a structure in which a magnification conversion optical system can be inserted into and removed from a predetermined interval in the zoom lens. Japanese Patent No. 3,513,264 proposes a zoom lens in which defocus due to temperature variation is suppressed while reducing the second-order spectrum of the axial chromatic aberration even when the magnification conversion optical system is inserted.
A positive-lead type four-unit zoom lens having a structure described above may obtain a high zoom ratio relatively easily. In order to obtain high optical performance in this four-unit zoom lens, it is important to correct the lateral chromatic aberration at the wide-angle end and the axial chromatic aberration at the telephoto end appropriately. It is easy to appropriately correct the lateral chromatic aberration and the axial chromatic aberration if an optical material having extraordinary dispersion is used.
However, in general, an organic material such as a resin and plastic having extraordinary dispersion has a large value dn/dT of refractive index variation due to temperature variation, which is approximately 10 to 200 times larger than that of glass. Therefore, when an optical element made of an organic material having strong refractive power is used in order to obtain a chromatic aberration correction effect, it is important to reduce defocus (image plane shift) in an optical system due to ambient temperature variation. In order to reduce the defocus in an optical system due to ambient temperature variation, it is necessary to maintain a small back focus variation amount due to ambient temperature variation.